Quantum photonic integrated circuit (QPIC)

A quantum photonic integrated circuit is a PIC whose job is quantum optics: producing single photons or entangled pairs, routing them through programmable interference, and detecting them with single-photon resolution. The standard source is spontaneous four-wave mixing in a silicon ring resonator or spontaneous parametric down-conversion in a periodically poled waveguide; the processing layer is a mesh of Mach-Zehnder interferometers with thermo-optic or electro-optic phases; detection is increasingly an on-chip superconducting nanowire detector, which drags the whole assembly to cryogenic temperatures.

What separates a QPIC from a classical PIC is where the specifications bind. Loss is not a power budget but a success probability: every 3 dB halves the rate at which multi-photon experiments produce an event at all, which is why heralded sources, low-loss silicon nitride routing, and high-efficiency detectors are the whole game. Phase stability substitutes for the interferometer table, and indistinguishability between photons from separate sources, measured as Hong-Ou-Mandel visibility, becomes a fabrication-uniformity spec. The field's commercial wings, photonic quantum computing of the fusion-based sort, chip-based QKD transmitters, and quantum-enhanced sensing, all reduce to driving those three numbers, loss, visibility, and detector efficiency, simultaneously.

For a characterization lab the honest note is that most QPIC test is classical photonics with stricter bookkeeping: insertion loss per element measured by cutback-style test structures, ring Q and coupling extraction, phase-shifter transfer functions and crosstalk, all performed before any quantum light is involved. The quantum measurements, coincidence rates, heralding efficiency, visibility, then certify what the classical numbers predicted.

References: J. Wang, F. Sciarrino, A. Laing, M. G. Thompson, Nat. Photonics 14, 273 (2020); G. Moody et al., J. Phys. Photonics 4, 012501 (2022).